Upon discovery, lactulose had been prepared from an alcoholic solution in the form of anhydrous lactulose crystal (hereinafter sometimes referred to as "anhydride") [Journal of American Chemical Society, Vol. 52, p. 2101, 1930]. Since then, lactulose has been believed to be prepared by forming an anhydride. More recently, it was reported that lactulose trihydrate crystal (hereinafter sometimes referred to as "hydrate") was available from an aqueous solution [Carbohydrate Research, Vol. 6, p. 29, 1992].
As lactulose is a substance useful as a growth promoting factor of Bifidobacterium and a medical drug for hepatic encephalopathy, various manufacturing methods have so far been developed.
When manufacturing lactulose, obtaining a hydrate from an aqueous solution is preferable to obtaining an anhydride from an alcoholic solution, from the points of view of simplicity, safety and economic advantages.
Lactulose is on the other hand commercially produced in the form of a syrup or powder. For the convenience in handling, commercial production in a powdery form is preferable. Lactulose is not however suitable for producing into a powdery product because of a high water-solubility, and particularly such a low melting point as 68.1.degree. C. of the hydrate, resulting in melting at a temperature of about 58.degree. to 60.degree. C.
The anhydride has in contrast such a high melting point as 169.degree. C. and is stable even in a powdered state, leading to an important value of use. There is therefore an increasing demand for a manufacturing method which is safe and simple as that of the hydrate.
From such a point of view as described above, the present inventors invented a method for converting a hydrate into an anhydride, and filed an application for a patent under the title of a method for manufacturing crystalline lactulose anhydride (Japanese Patent Application No. 03-272,841; Japanese Patent Provisional Publication No. 05-111,400; hereinafter referred to as the "prior application"). The method of this prior application comprises the steps of drying crystalline lactulose trihydrate dried at the room temperature, under the atmospheric pressure and at a temperature within a range of from 45.degree. C. to the melting point, and then drying same at a temperature of up to 80.degree. C.
The method of the prior application has made it possible to eventually obtain lactulose anhydride from an aqueous solution, not from an alcoholic solution, as described above.
Even in this excellent method of the prior application, however, there still remain problems to be solved before application in an industrial scale. More specifically, the method of the prior application has drawbacks in that, since crystalline lactulose trihydrate is dried by, for example, the fluidized-bet drying method, crystalline lactulose fines scatter, resulting in a decrease in the final product yield, and that the fluidized-bet drier is large in size and high in cost.